Warrants	6 Months Ended
Jun. 30, 2019
Warrants And Rights Note Disclosure [Abstract]
Warrants	7. Warrants Upon the closing of the IPO, on May 4, 2015, the Company issued to the representative of the underwriters as additional compensation a warrant to purchase an aggregate of 82,500 shares of the Company’s common stock. The warrant was exercisable for cash or on a cashless basis at a per share exercise price equal to $10.00 commencing on April 28, 2016, one year following the date of the prospectus filed with the SEC relating to the IPO, and expiring on April 28, 2020. The warrant was exercised in full during the year ended December 31, 2018. The warrant also provided for registration rights upon request, under certain circumstances. The piggyback registration right provided in connection with the warrant was to terminate on April 28, 2022. On April 13, 2016, pursuant to an underwritten public offering (the “April 2016 Offering”), the Company sold 7,500,000 shares of its common stock and warrants to purchase up to 7,500,000 shares of its common stock at a public offering price of $1.25 per share of common stock and related warrant. The warrants have an exercise price of $1.50 per share of common stock, were immediately exercisable upon issuance and will expire on April 13, 2021. Additionally, on April 13, 2016, the underwriters for the April 2016 Offering partially exercised the over-allotment option for warrants to purchase an additional 1,125,000 shares of the Company’s common stock at a public offering price of $0.01 per warrant to purchase a share of common stock. As of June 30, 2019, warrants to purchase an aggregate of 4,168,238 shares were outstanding and warrants to purchase an aggregate of 116,603 and 356,363 shares were exercised during the three and six months ended June 30, 2019, respectively. On April 13, 2016, pursuant to the terms of the Loan and Security Agreement, the Company issued to Ligand a warrant to purchase up to 960,000 shares of the Company’s common stock (the “Ligand Warrant”). The Ligand Warrant has an exercise price of $1.50 per share of Company common stock, was immediately exercisable upon issuance (subject to a limitation on exercise to the extent that any exercise thereof would increase Ligand’s beneficial ownership of the Company’s common stock to greater than 49.9%) and expires on April 13, 2021. The Ligand Warrant was issued to Ligand as a part of the repayment of $1.2 million of the Company’s obligation under the secured convertible promissory note issued to Ligand pursuant to the loan and security agreement with Ligand. On June 14, 2017, the Company entered into a securities purchase agreement (the “Securities Purchase Agreement”), with certain accredited investors (the “Purchasers”), pursuant to which the Company sold an aggregate of 3,749,783 shares (the “Shares”), of its common stock in a registered direct offering (the “Registered Direct Offering”). The Shares were offered by the Company pursuant to the Shelf Registration Statement. In a concurrent private placement, the Company also agreed, pursuant to the Securities Purchase Agreement, to issue and sell to each of the Purchasers a warrant to purchase 0.75 shares of common stock (the “Warrants”), for each share of common stock purchased by a Purchaser in the Registered Direct Offering (the “Private Placement”) and, together with the Registered Direct Offering, (the “Offerings”). On June 14, 2017, pursuant to the terms of the Securities Purchase Agreement, the Company sold Shares and the Warrants to purchase up to 2,812,337 shares of its common stock to the Purchasers. The combined purchase price for one Share and one Warrant to purchase 0.75 shares of common stock in the Offerings, was $1.15. The closing of the Offerings occurred on June 19, 2017. The Warrants have an exercise price of $1.30 per share, subject to adjustment as provided therein, and will be exercisable beginning on December 19, 2017 through December 19, 2022. Each holder of a Warrant will not have the right to exercise any portion of its Warrant if the holder, together with its affiliates, would beneficially own in excess of 4.99% of the number of shares of common stock outstanding immediately after giving effect to such exercise (the “Beneficial Ownership Limitation”); provided, however, that upon 61 days’ prior notice to the Company, the holder may increase the Beneficial Ownership Limitation; however, in no event shall the Beneficial Ownership Limitation exceed 9.99%. The exercise price and number of shares of common stock issuable upon the exercise of the Warrants will be subject to adjustment in the event of any stock dividends and splits, reverse stock split, recapitalization, reorganization or similar transaction, as described in the Warrants. If a registration statement covering the issuance or resale of the shares of common stock issuable upon exercise of the Warrants is not available for the issuance or resale, as applicable, the Purchasers may exercise the Warrants by means of a “cashless exercise.” On January 16, 2018, the resale registration statement on Form S-1 (File No. 333-222202) that the Company filed related to the 1,987,337 shares subject to unexercised warrants was declared effective by the SEC. As of June 30, 2019, warrants to purchase an aggregate of 885,395 shares were outstanding and no warrants were exercised during the three and six months ended June 30, 2019.

Warrants

6 Months Ended

Jun. 30, 2019

Warrants And Rights Note Disclosure [Abstract]

7. Warrants

Upon the closing of the IPO, on May 4, 2015, the Company issued to the representative of the underwriters as additional compensation a warrant to purchase an aggregate of 82,500 shares of the Company’s common stock. The warrant was exercisable for cash or on a cashless basis at a per share exercise price equal to $10.00 commencing on April 28, 2016, one year following the date of the prospectus filed with the SEC relating to the IPO, and expiring on April 28, 2020. The warrant was exercised in full during the year ended December 31, 2018. The warrant also provided for registration rights upon request, under certain circumstances. The piggyback registration right provided in connection with the warrant was to terminate on April 28, 2022.

On April 13, 2016, pursuant to an underwritten public offering (the “April 2016 Offering”), the Company sold 7,500,000 shares of its common stock and warrants to purchase up to 7,500,000 shares of its common stock at a public offering price of $1.25 per share of common stock and related warrant. The warrants have an exercise price of $1.50 per share of common stock, were immediately exercisable upon issuance and will expire on April 13, 2021. Additionally, on April 13, 2016, the underwriters for the April 2016 Offering partially exercised the over-allotment option for warrants to purchase an additional 1,125,000 shares of the Company’s common stock at a public offering price of $0.01 per warrant to purchase a share of common stock. As of June 30, 2019, warrants to purchase an aggregate of 4,168,238 shares were outstanding and warrants to purchase an aggregate of 116,603 and 356,363 shares were exercised during the three and six months ended June 30, 2019, respectively.

On April 13, 2016, pursuant to the terms of the Loan and Security Agreement, the Company issued to Ligand a warrant to purchase up to 960,000 shares of the Company’s common stock (the “Ligand Warrant”). The Ligand Warrant has an exercise price of $1.50 per share of Company common stock, was immediately exercisable upon issuance (subject to a limitation on exercise to the extent that any exercise thereof would increase Ligand’s beneficial ownership of the Company’s common stock to greater than 49.9%) and expires on April 13, 2021. The Ligand Warrant was issued to Ligand as a part of the repayment of $1.2 million of the Company’s obligation under the secured convertible promissory note issued to Ligand pursuant to the loan and security agreement with Ligand.

On June 14, 2017, the Company entered into a securities purchase agreement (the “Securities Purchase Agreement”), with certain accredited investors (the “Purchasers”), pursuant to which the Company sold an aggregate of 3,749,783 shares (the “Shares”), of its common stock in a registered direct offering (the “Registered Direct Offering”). The Shares were offered by the Company pursuant to the Shelf Registration Statement. In a concurrent private placement, the Company also agreed, pursuant to the Securities Purchase Agreement, to issue and sell to each of the Purchasers a warrant to purchase 0.75 shares of common stock (the “Warrants”), for each share of common stock purchased by a Purchaser in the Registered Direct Offering (the “Private Placement”) and, together with the Registered Direct Offering, (the “Offerings”). On June 14, 2017, pursuant to the terms of the Securities Purchase Agreement, the Company sold Shares and the Warrants to purchase up to 2,812,337 shares of its common stock to the Purchasers. The combined purchase price for one Share and one Warrant to purchase 0.75 shares of common stock in the Offerings, was $1.15. The closing of the Offerings occurred on June 19, 2017. The Warrants have an exercise price of $1.30 per share, subject to adjustment as provided therein, and will be exercisable beginning on December 19, 2017 through December 19, 2022. Each holder of a Warrant will not have the right to exercise any portion of its Warrant if the holder, together with its affiliates, would beneficially own in excess of 4.99% of the number of shares of common stock outstanding immediately after giving effect to such exercise (the “Beneficial Ownership Limitation”); provided, however, that upon 61 days’ prior notice to the Company, the holder may increase the Beneficial Ownership Limitation; however, in no event shall the Beneficial Ownership Limitation exceed 9.99%. The exercise price and number of shares of common stock issuable upon the exercise of the Warrants will be subject to adjustment in the event of any stock dividends and splits, reverse stock split, recapitalization, reorganization or similar transaction, as described in the Warrants. If a registration statement covering the issuance or resale of the shares of common stock issuable upon exercise of the Warrants is not available for the issuance or resale, as applicable, the Purchasers may exercise the Warrants by means of a “cashless exercise.” On January 16, 2018, the resale registration statement on Form S-1 (File No. 333-222202) that the Company filed related to the 1,987,337 shares subject to unexercised warrants was declared effective by the SEC. As of June 30, 2019, warrants to purchase an aggregate of 885,395 shares were outstanding and no warrants were exercised during the three and six months ended June 30, 2019.